The present invention relates to a lamp used for a gas-filled lamp, such as a halogen lamp, a single-tube high-intensity discharge lamp and a double-tube high-intensity discharge lamp.
A molybdenum foil is generally provided as a conductor at a sealing portion of a lamp used for a halogen lamp, a single-tube high-intensity discharge lamp, a double-tube high-intensity discharge lamp having an outer housing formed of quartz, and the like. The outer covering of the sealing portion in this kind of lamp has been formed of a transparent material such as quartz. The distal end of the molybdenum foil in the sealing portion away from the center of the bulb is exposed to the air.
In the conventional lamp having the above-mentioned structure, when the lamp is burning, the temperature of the molybdenum foil is raised abruptly by radiation heat from the lamp, conduction heat transferred through the sealing portion, heat generation due to the resistance of the molybdenum foil itself by the passage of electric current, and the like.
Since the distal end of the molybdenum foil in the sealing portion away from the center of the bulb is heated at a high temperature in the air as described above, the molybdenum foil is apt to be oxidized. When this kind of lamp is lighted for a considerable period of time, the molybdenum foil is oxidized, and the sealing portion is deteriorated, whereby the service life of the lamp is shortened.
In order to prevent the above-mentioned molybdenum foil from being oxidized, it was necessary to lower the temperature of the sealing portion to 350.degree. C. or less, while the lamp is burning.
However, in such a conventional lamp consuming high power, its sealing portion is heated at a high temperature. Therefore, it was difficult to lower the temperature of the molybdenum foil to 350.degree. C. or less.
In order to solve the above-mentioned problems, various cooling means have been taken for the conventional lamp. For example, its caps are provided with heat radiation fins, or its sealing portions are extremely lengthened so as to locate the molybdenum foils at the ends of the sealing portions away from the light-emitting portion.
It is necessary to have the above-mentioned cooling means for a lamp consuming high power. In particular, long sealing portions have been used for a lamp consuming 1800 W or more.
However, since the cap of the conventional lamp provided with heat radiation fins is complicated in shape, the production cost of the cap increases. In addition, the lamp provided with extremely lengthened sealing portions is difficult to produce and becomes large.
These problems have raised the production cost of the lamp even having the configuration to keep the sealing end portions at relatively low temperature. In particular, the pinch sealing method being advantageous for reducing the production cost cannot be used for such a lamp having extremely lengthened sealing portions.